This invention relates to a method for bonding a metal foil to a ceramic substrate. More particularly, this invention relates to such method utilizing, as a bonding agent, a eutectic composition of the metal and its oxide that is derived from an oxide film that is preformed onto the substrate prior to application of the foil. In one aspect of this invention, a copper foil is bonded to a ceramic substrate utilizing a preformed cuprous oxide layer.
In the manufacture of electronic components and the like, it is known to bond a copper foil to a ceramic substrate using a eutectic bonding agent. For this purpose, the surface of the copper foil is initially oxidized by heating in a atmosphere having a controlled low oxygen content to form a layer of cuprous oxide compound, Cu.sub.2 O. In appropriate proportions, copper and cuprous oxide form a low melting eutectic oxide mixture containing 0.34 weight percent oxygen and having a melting point of about 1065.degree. C., significantly less than metallic copper. The foil is placed with the cuprous oxide film lying against the substrate. The arrangement is heated above the eutectic melting point, whereupon incipient melting occurs between the copper metal and the cuprous oxide to form a eutectic liquid phase. This continues until the cuprous oxide is consumed. The resulting liquid phase wets the ceramic surface and, upon cooling, resolidifies to bond the copper foil to the substrate.
Although this eutectic bonding method has been generally satisfactory in forming a tight bond between the metal foil and the nonmetallic substrate, it nevertheless suffers from several deficiencies. Oxidation of the foil must be controlled not only to form the desired cuprous oxide compound, as opposed to the greater oxidized cupric oxide, CuO, but to limit the thickness of the film to a few Angstroms so as not to jeopardize the desired metallic properties of the bulk of the foil. The oxidized film needs to be suitably thick to produce sufficient liquid phase to assure wetting of the surface that is essential to forming a strong bond. On the other hand, a film that is too thick produces excess liquid phase, which consumes a greater proportion of the metal foil and tends to spread onto unwanted regions. Difficulty also arises in positioning the foil with the oxide film accurately against the prescribed region, which is particularly significant for foils having complex configurations such a lines of an electrical circuit. Thus, there remains a need for an improved method for eutectic oxide bonding of copper or other metal foil to a nonmetallic substrate.